1. Field of the Invention
The present invention relates to an air/fuel ratio control device for an internal combustion engine having, for example, a variable valve timing control mechanism capable of freely changing intake valve timing, and capable of executing purge control.
2. Description of Related Art
A conventional air/fuel ratio control device for an internal combustion engine is disclosed in Japanese Patent Application Laid-Open No. 7-83096. The application describes a device in which a purge flow rate is controlled with respect to an intake amount. Also, purge fuel concentration is detected based on an air/fuel ratio feedback value (coefficient) during fuel purge, and the fuel injection amount is corrected.
In the meantime, when the above-described technology is applied to an electronic control type fuel injection system referred to as an "L-Jetronic" system, where a fuel injection amount supplied to an internal combustion engine is controlled by using an air flow meter, it is necessary to calculate an opening degree of a purge valve indirectly from an intake amount, an intake charge efficiency or the like to secure a desired purge flow amount. In this case, according to an internal combustion engine La having a variable valve timing control mechanism, when the valve timing is changed, the intake amount or the intake charge efficiency is changed. Accordingly, the desired purge flow amount cannot be accurately controlled. Thus, the air/fuel ratio control performance is deteriorated, thereby reducing vehicle drivability and vehicle emission performance.